(1) Field of the Invention
The present invention relates to the field of semiconductor wafers processing. More specifically, the present invention relates to an assembly for holding a wafer subject to a process.
(2) Description of the Related Art
Semiconductor wafers are typically subject to a multitude of processes such as etching, thin film deposition, etc. Typically, in a plasma etching process, semiconductor wafers are inserted in a vacuum chamber where they are subject to plasma discharge. FIG. 1 shows a prior art plasma etching apparatus wherein the lower electrode 2 supports a wafer 4, subject to plasma etching. The wafer 4 is fixed in place, on the lower electrode 2, by a mechanical holding means such as a clamp 6. A gas container 8 including a heat transfer gas such as helium, by way of non-limiting example, is fed upwardly, relatively to this figure, to an aperture 10 of the lower electrode 2. The gas is then introduced into the volume defined between a top surface 12 of the lower electrode 2 and a bottom surface 14 of the wafer 4.
Helium is typically introduced between the wafer 4 and the lower electrode 2 for transferring heat between the wafer 4 and the lower electrode 2. The lower electrode 2 is maintained at a colder temperature by recirculating a cooling fluid via a channel 11 routed within the lower electrode 2. Conversely, the lower electrode 2 can be maintained at a higher temperature, when such operation is required, by circulating a fluid at a higher temperature via the channel 11. Accordingly, the helium gas provides a medium for heat transfer between the wafer 4 and the lower electrode 2.
Generally, the etching of the wafer 4 is performed by applying a source of RF power to the lower electrode 2 and coupling the walls 19 of the process chamber 23 to the ground. The etching of wafer 4 can be equally performed by applying the source of RF power to an upper electrode 24 while coupling the lower electrode 2 to the ground. When RF power is applied across these electrodes, i.e., the lower electrode 2 and the walls of the walls 19, a plasma discharge can be generated therebetween. In a preferred plasma etching process, fluorine-based plasma can etch a layer of Tungsten (W) of the wafer 4 by having the fluorine ions, disassociated in the plasma, react with the Tungsten layer of the wafer. The fluorine ions"" reaction with the layer of Tungsten of the wafer causes the Tungsten layer to be etched. A Tungsten hexa-fluoride compound is formed as a result of the reaction. The reaction-between the plasma and the wafer, as well as the power dissipated to the wafer surface, causes the temperature of the wafer to rise. As the temperature of the wafer increases, the selectivity to the underlying layers of the wafer decreases causing the etching to become uncontrollable. Because the etching process is temperature sensitive, it is desirable to provide a mechanism for controlling the temperature of the wafer as well as for maintaining a substantially uniform temperature across the wafer.
The plasma etching process, generally, requires an inert gas, such as helium, to be introduced via the through hole passage 10 of the lower electrode 2 into the volume between the lower electrode 2 and the wafer 4. The heat transfer gas, however, is not limited to an inert gas, but can be any gas which can transfer heat with minimum heat loss and does not easily react with the process plasma in the process chamber 23. Typically, the heat transfer gas is maintained at a pressure which may vary between 0.1 and 15 Torr. Preferably 10 Torr is supplied from the heat transfer gas source into the volume between the wafer 4 and the lower electrode 2. The pressure of the heat transfer gas typically exceeds the pressure in the process chamber. Accordingly, the wafer 4 will bow as shown in this figure, thereby causing the center 18 of this wafer to be lifted further away, from the top surface 12 of the lower electrode 2, than the periphery 16 of the wafer. The bow-like shape of wafer 4, with the periphery 16 dose to the lower electrode 2, and the center 18 spaced apart from the top surface 12 of the lower electrode 2, contributes to non-uniform heat transfer between the lower electrode 2 and the wafer 4, as more heat is dissipated towards the periphery 16 of the wafer 4 and less heat is dissipated towards the center 18 of the wafer 4.
Additionally, the configuration shown in FIG. 1 poses considerable problems with respect to the sealing of wafer 4 against the lower electrode 2. Since both the wafer 4 and the lower electrode 2 are typically made of a hard material, the surfaces, of the wafer and of the lower electrode, which are in contact with each other, may not be able to provide adequate sealing due to the poor conformability of hard surfaces. Consequently, minor variations in the surface of the wafer and of the lower electrode may cause the helium gas to leak out, thereby causing fluctuations in the pressure of the helium.
It is desirable to provide for a plasma etching apparatus having a lower electrode supporting a wafer wherein a gas introduced between the lower electrode and the wafer produces substantially uniform heat transfer across the wafer. Additionally, it is desirable to provide for improved sealing between the wafer and the lower electrode.
The present invention provides an assembly for holding a substrate. The substrate has a first surface, a second surface opposite the first surface, and an outer peripheral portion. The assembly includes a holding body having a support surface for supporting the substrate. The holding body has an aperture for passing therethrough a gas having a thermal conductivity. The assembly further includes a heat transferring seal having a first surface for frictionally engaging the second surface of the substrate. The heat transferring seal has a second surface, opposite the first surface, for frictionally engaging the support surface of the holding body. The heat transferring seal further has an inner peripheral portion defining an opening for receiving the gas. The heat transferring seal also has a thermal conductivity, closely matched with the thermal conductivity of the gas, for providing substantially uniform heat transfer across the substrate.